Recently, researchers in England discovered that simply rinsing your mouth with a sports drink may fight fatigue. In the experiment, which was published online in February in the Journal of Physiology, eight well-trained cyclists completed a strenuous, all-out time trial on stationary bicycles in a lab. The riders were hooked up to machines that measured their heart rate and power output. Throughout the ride, the cyclists swished various liquids in their mouths but did not swallow. Some of the drinks contained carbohydrates, the primary fuel used during exercise. The other drinks were just flavored, sugar-free water.

By the end of the time trials, the cyclists who had rinsed with the carbohydrate drinks — and spit them out — finished significantly faster than the water group. Their heart rates and power output were also higher. But when rating the difficulty of the ride, on a numerical scale, their feelings about the effort involved matched those for the water group.

In a separate portion of the experiment, the scientists, using a functional M.R.I., found that areas within the brain that are associated with reward, motivation and emotion were activated when subjects swished a carbohydrate drink. It seems that the brains of the riders getting the carbohydrate-containing drinks sensed that the riders were about to get more fuel (in the form of calories), which appears to have allowed their muscles to work harder even though they never swallowed the liquid.

The role of the brain in determining how far and hard we can exercise — its role, in other words, in fatigue — is contentious. Until recently, most researchers would have said that the brain played little role in determining how hard we can exercise. Muscles failed, physiologists thought, because of biochemical reactions within the muscles themselves. They began getting too little oxygen or were doused with too much lactic acid or calcium. They stiffened and seized.

But there are problems with the idea that fatigue involves only the muscles. “We know that people speed up at the end of exercise,” says Ross Tucker, a researcher with the Sports Science Institute of South Africa, who has extensively studied fatigue in athletes. “If calcium” or other biochemical changes in the muscles “caused muscle failure, this would be impossible at the end, when these changes are at their greatest levels.”

Instead, he and many (but not all) physiologists now believe that exhaustion isn’t just in the muscles but also involves the brain. “What we now think is that the muscle isn’t acting on its own,” he says. “There’s an interplay of central processing and muscular exertion.” From the outset of exercise, “the brain asks for and gets constant feedback from the muscles and other systems especially about body temperature” and checks on “how are things going,” says Carl Foster, a professor in the department of exercise and sports science at the University of Wisconsin in La Crosse. Through mechanisms that aren’t fully understood, the brain tracks and calibrates the amount of fuel that is in the muscles, as well as the body’s core temperature. As the amount of fuel drops and the temperature rises, the brain decides that some danger zone is being approached. It starts reducing “the firing frequency of motor neurons to the exercising muscle, leading to a loss of force production,” says Ed Chambers, a researcher at the School of Sport and Exercise Sciences at the University of Birmingham in England and an author of the carbohydrate-drinks study. In other words, the mind, recognizing that the body may be going too hard, starts sending fewer of the messages that tell the muscles to contract. The muscles contract less frequently and more feebly. In a sensation familiar to anyone who exercises, your legs die beneath you.

The mental choreography of fatigue is intricate, involving messages sent not only from the brain to the straining muscles but also to various areas within the mind as well. Data from some recent brainwave studies of athletes showed that during long, hard exercise, there’s often a moment when portions of the brain become “de-aroused,” Foster says. “It’s similar to depression,” he adds, and plays out in motivation. You begin to wonder why in the world you’re running, swimming or pedaling so hard. You slow down.

“I think the training effect of this theory is potentially very profound,” Tucker says. “Training is no longer simply an act of getting the muscles used to lactate or teaching the lungs how to breathe harder.” It’s also about getting your brain to accept new limits by pushing yourself, safely. “Once your brain recognizes that you’re not going to damage yourself,” Foster says, “it’ll be happy to let you go.”